Russian forces have left the site of the Chernobyl nuclear plant early on Friday amid reports of soldiers receiving "significant doses" of radiation. 

Control over the site was returned to Ukraine, but authorities are braced for renewed attacks in the east.

Ukraine’s state power company, Energoatom, claimed soldiers were subject to radiation from digging trenches in the exclusion zone around the closed plant. 

The International Atomic Energy Agency said it had been informed by Ukraine that the Russian forces at the site of the world’s worst nuclear disaster had transferred control of it in writing to the Ukrainians.

READ MORE: Men offering Ukrainian women refuge for sex warns Glasgow charity

There was no immediate comment from the Kremlin about soldiers being impacted by radiation, and the IAEA said it had not been able to confirm the reports of Russian troops receiving high doses.

The exchange of control at Chernobyl happened amid growing indications the Kremlin is using talk of de-escalation in Ukraine as cover to regroup, resupply its forces and redeploy them in the east. 

Ukrainian president Volodymyr Zelensky warned that Russian withdrawals from the north and centre of the country were just a military tactic to build up forces for new powerful attacks in the south east.

A new round of talks between the countries was scheduled on Friday, five weeks into a conflict that has left thousands dead and driven four million Ukrainians from the country.

“We know their intentions,” Mr Zelensky said in his nightly video address to the country.

“We know that they are moving away from those areas where we hit them in order to focus on other, very important ones where it may be difficult for us.”

“There will be battles ahead,” he added.

Russians also blocked another aid mission to the besieged city of Mariupol, which has suffered heavy shelling over the duration of the conflict. 

A total of 45 buses were stopped while trying to evacuate people after an agreed limited ceasefire. 

Only 631 people were able to get out of the city in private cars, according to the Ukrainian government.

Russian forces also seized 14 tons of food and medical supplies in a dozen buses that were trying to make it to Mariupol, deputy prime minister Iryna Vereshchuk said.

Meanwhile, in what would be the first attack of its kind, if confirmed, the governor of the Russian border region of Belgorod accused Ukraine of flying helicopter gunships into Russian territory early on Friday and striking an oil depot.

The depot, a facility run by Russian energy giant Roseneft about 21 miles from the border, was set ablaze by the attack that left two people injured, according to a post by Vyacheslav Gladkov on Telegram.

“The fire at the oil depot occurred as a result of an air strike from two helicopters of the armed forces of Ukraine, which entered the territory of Russia at a low altitude,” the governor wrote on the messaging app.

It was not immediately possible to verify the claim or images that were circulating on social media.